Coatings for columbium base alloys



Dec. 20, 1966 E. F. BRADLEY ETAL 3,293,068

COATINGS FOR COLUMBIUM BASE ALLOYS Filed Aug. 19. 1963 Mixed-phase zone(MnSi M51 Disilicide zone (M51 Subsilicide zone Alloy substrateSTRUCTURE OF A 1 :2 Mn:Si RATIO COATING 500x MAGNIFICATION 3mm ELIHU F.BRADLEY EDWIN s. BARTLETT HORACE R. OGDEN as, ROBERT 1. JAFFEE Irons,Birch, Swindler & McKie 3,293,068 COATINGS FOR COLUMBIUM BASE ALLOYSElihu F. Bradley, West Hartford, Conn., and Edwin S.

This invention relates to novel coatings for columbium and columbiumbase alloys that will protect the base metal or alloy from oxidation invery high temperature environments and to a method for creating suchcoatings. More particularly, this invention relates to modified silicidecoatings for columbium and its alloys in which the coatings are createdby vapor deposition, flame or plasma torch spraying, slurry applicationtechniques, electrophoretic deposition, hot pressure bonding, and thelike, and to a method for obtaining vapor deposition of modifiedsilicide coatings on columbium base materials to produce a protectivelayer over the base metal of the alloys that provides an oxidationresistant coating for the base metal at very high temperatures, such as,for example, temperatures up to at least 2200 F.

The principal limitation in gas turbine technology today is the maximumturbine inlet temperature. The turbine inlet temperature is, in turn,set by the temperature that the turbine vanes and blades are able towithstand without danger of failure. Formerly, the best available hightemperature alloys were nickel and cobalt base superalloys, but criticalstructural components, such as turbine vanes and blades constructed fromsuch alloys, are limited to maximum operating temperatures of between1600 and 1900 F.

For many years it has been generally known that the high temperaturestrength properties of metals are closely related to their meltingpoints. Thus, metals having high melting points also tend to have hightemperature strength potentials.

The need for structural materials for service at temperatures in excessof those obtainable with existing materials of construction, such as,nickel and cobalt alloys, has stimulated interest in the metals havingthe highest melting points, or the refractory metals, particularly,chromium, columbium, molybdenum, and tungsten. Until recently molybdenumwas considered the chief prospect for such usage. However, at the hightemperature service conditions needed, molybdenum oxidizes at acatastrophic rate, principally because molybdenum oxide is volatile atelevated temperatures.

As an alloy base for high temperature service, columbium offers promise,and considerable interest has been directed to its use as a structuralalloy base for applications in high-temperature environments. Among thetechnically most important physical qualities of columbium as an alloybase are its high melting temperature (4380 F.) and its lowneutron-capture cross-section. Columbium is, therefore, potentiallyuseful for such applications as fast aircraft, space flight vehicles,and nuclear reactors.

Further, columbium is inherently a soft, ductile, readily fabricablematerial, and although its melting temperature is about 4380 F. purecolumbium becomes too weak for practical structural uses at temperaturesabove 1200 F. Columbium is also a very reactive metal in that itdissolves large quantities of oxygen and probably nitrogen, on exposureto atmospheres containing even small amounts of these elements at modesttemperatures.

The history of columbium alloy technology has demonstrated theincompatability of achieving oxidation resistance and high-temperaturestrength through alloying alone. Since the major fields of utility forcolumbium base alloys depend largely upon retention of high-temperaturestrength in the alloys, it is apparent that useful classes of columbiumalloys will demand coatings for protection when used in their normalhigh temperature oxidizing environments.

Coatings of typical classes used for columbium base substrates are hardand brittle and are thus subject to cracking or other failure atlocalized sites. In contrast to molybdenum which oxidizescatastrophically, the oxide of columbium does not volatilize, and it isthus potentially possible to prevent oxygen attack on columbium bycoating the metal, and should premature localized coating failure occur,to restrict the failure and oxygen attack to the localized site. Furtheradvantages offered by columbium base alloys as compared with molybdenumbase alloys are the columbium alloys are relatively more ductile andworkable at low temperatures and columbium has a lower density thanmolybdenum.

A particularly important potential area of use for columbium base alloysas dictated by economic and technological considerations is inapplications of such alloys requiring exposure to oxidizing environmentsat temperatures up to about 2200 F. (a temperature that clearlyestablishes utility for columbium base alloys), with the concomitantrequirement that the alloys must be able to resist strong stresses forappreciable periods of time at such high temperatures. About 1000 F. isthe maximum temperature to which high-stress rupture strength columbiumbase alloys may be subjected for extended times in the uncoatedcondition without serious oxidation, and at temperatures above 1000 F.the oxidation problem becomes acute.

The art has previously recognized oxidation-resistant intermetalliccoatings that exhibit particular potential for protecting, refractorymetals (i.e., columbium, molybdenum, tantalum, and tungsten) fromoxidation at temperatures up to about 3000 F. In general, the moreeffective of these intermetallic coatings are further classified assilicides, aluminides and beryllides. In considering coatings for therefractory metals, both coating and substrate materials are important tothe performance of the coated systems. For example, silicide coatingsover columbium and molybdenum may perform very differently, with thedifference in performance attributable to the substrate rather than thecoating type. As an additional confirmation of the importance of thesubstrate, some species of coatings that are reliably protective over,for example, tantalum are ineffective over columbium, because they aresusceptible to premature localized defect failures at high temperatures.

Several methods, such as, flame or plasma torch sprayings, slurryapplication techniques, electrophoretic deposition, hot pressurebonding, or vapor deposition, may be used for applying intermetalliccoatings to columbium base alloys. A vapor deposition process that canbe employed advantageously is the so-called pack cementation process, inwhich the object to be coated is surrounded by a particulate packmixture containing, for example, the metal to be reacted with, ordeposited upon, the object to be coated (e.g., silicon aluminum,beryllium), an activator or energizer (usually a halide salt, such as,NaCl, KF, NH I, NH CI, and the like), and an inert filler material(e.g., A1 SiO BeO, MgO, and the like). This mixture, held in a suitablecontainer (steel box, graphite boat, or refractory oxide crucible, forexample), is then heated to a desired coating temperature in aprescribed atmosphere and held for a length of time sufiicient toachieve the desired coating. When conducted properly, the packcementation process will result in controlled-thickness coatings on, forexample, columbium, the major proportions of which will be, for example,CbAl (aluminide), or CbSi (silicide), and the like.

The more favorable coatings (aluminides, silicides, beryllides) forcolumbium possess certain intrinsic deficiencies, such as rapidoxidation failure at low temperatures (in the vicinity of 1300 F.) or athigh temperatures (greater than about 2000" F.). Perhaps the mostserious deficiency of existing coatings for columbium, however, is theirpropensity toward failure at localized sites. For technological reasons,silicide coatings on columbium and its structural alloys are lesssusceptible to localized failures than are aluminide or beryllidecoatings, and are thus of primary interest. Silicide coatings onstructural columbium alloy substrates, however, are prone to rapidconsumption by oxidation at low (about 1300 F.) temperatures (thischaracteristic of silicide coatings is sometimes termed the silicidepest phenomenon) and at high (about 2000 F. or higher) temperatures.Modification of silicide coatings is highly desirable toimpart to themsuflicient longevity to give them a utility that they do not normallypossess.

Copending application Serial No. 65,962, filed October 31, 1960, nowabandoned, discloses and claims a class of fabricable, ductile,stress-rupture resistant columbium base alloys that will readily fulfillthe structural requirements for use at high temperatures up to at least2500" F. Typical of this latter class of alloys is the compositionCb-20Ta-15W-5Mo (additions expressed in percent by Weight).

In view of the foregoing, it is a primary object of this invention toprovide a coating composition that will protect such stress-ruptureresistant alloys from the effects of oxidation at temperatures up to atleast about 2200 R, and that will achieve a modified silicide coatingthat is highly resistant to failure at localized sites.

Further objects of this invention are to provide a coating for columbiumand columbium base alloys that in addition to providing resistance tosimple thermal oxidation will also be protective under other reasonablyexpected conditions of use, and to this end, the protective coatings ofthis invention achieve good resistance to thermal cycling, thermalshock, formation of defects, and high velocity gas erosion.

Other objects of this invention are to provide for columbium andcolumbium base alloys (1) a coating that is self-healing, i.e., in thepresence of a major defect that exposes the alloy substrate to corrosiveenvironment, the

-of providing protection for exposures to high temperature oxidizingenvironments for times in excess of 100 hours at temperatures up to atleast about 2200 F (3) a coat- -ing that exhibits excellent resistanceto thermal shock failure; (4) a coating that displays excellent defectinsensitivity at both higher and lower temperatures than glass-formingtemperature for the coating; (5) a coating that demonstratesself-healing capabilities at temperatures above the glass-formingtemperature; and (6) a coating that achieves significant resistance tohigh velocity gas erosion.

Still further objects to this invention are to provide a method forcoating columbium and columbium base alloys with a modified columbiumsilicide using a vapor deposition (pack cementation) process that willachieve substantial uniformity of the coating and yield an essentiallyuniform coating on even intricately shaped parts and at the edges andcorners of parts.

Additional objects and advantages of the invention will be set forth inpart in the description that follows, and in part will be obvious fromthe description, or may be learned by practice of the invention, theobjects and advantages being realized and attained by means of thecompositions, methods, and processes particularly pointed out in theappended claims.

To achieve the foregoing objects, and in accordance with its purpose,this invention, in one of its forms, provides a new and improved articleof manufacture having good stress-rupture strength at high temperatures,high temperature oxidation resistance, and resistance to cyclic fatiguefailure, which article comprises a'core of 'metal selected from thegroup consisting of columbium and columbium base alloys, the articlehaving a thermal shock failure resistance, defect insensitive,self-healing surface layer or coating consisting essentially of amixture of manganese silicide and a silicide of the metal core. Thecoating may have a manganese to silicon ratio by weight of from about3:1 to 1:5, but gives particularly outstanding results when themanganese to silicon ratio by weight is from about 1:1 to 1:3, and inits most preferred form the manganese to silicon ratio of the coating byweight is found to be about 1:15.

More particularly described and in a preferred embodiment, the coatingof this invention comprises: a mixed phase surface zone or regionconsisting essentially of a mixture of manganese silicide and adisilicide of the substrate or metal core and a subzone or regionessentially underneath the surface Zone and consisting essentially of adisilicide of the substrate.

This invention also embraces as an article of manufacture, a refractorymetal body, comprising a substrate selected from the group consisting ofcolumbium and alloys thereof and having an exterior exposed layercomposed predominantly of a mixture of manganese silicide and a silicideof the metal body, the body being char- ,acterized by a goodstress-rupture strength at high temperatures and a high resistance tooxidation at temperatures up to at least about 2200 F.

The invention also includes within its scope a coated metal body havinggood stress-rupture strength at high temperatures, good oxidationresistance at high temperatures and resistance to cyclic fatiguefailure, which comprises a core of metal selected from the groupconsisting of columbium and its alloys, the body having a thermal shockfailure resistant, defect insensitive, self-healing oxidation resistantcoating, the coating comprising: a mixed phase surface zone consistingessentially of a mixture of manganese silicide and a disilicide of themetal core, an intermediate region essentially beneath the surface zoneconsisting essentially of a disilicide of the metal core, and adiffusion zone adjacent the metal core consisting essentially of atleast one subsilicide of the metal core. The coating in this form of theinvention may have a manganese to silicon ratio by weight of from about2:1 to 1:5, but gives particularly outstanding results when themanganese to silicon ratio by weight is from about 1:1 to 1:3, and inits most preferred form the manganese to silicon ratio of the coating byweight is about 111.5.

In accordance with its purpose, this invention includes a method ofproducing a coated metal article having re sistance to oxidation at hightemperatures and a good stress-rupture strength at high temperatures,which method comprises depositing a surface coating on a metalsubstrate, the substrate being a metal selected from a group consistingof columbium and alloys thereof, and

the coating consisting essentially of manganese silicide and a silicideof the metal substrate.

One embodiment of such method includes a two-cycle vapor depositionprocess of coating a fabricated base metal which process comprisessurrounding a base metal selected from the group consisting of columbiumand alloys thereof with a powdered pack of a finely ground sources ofmanganese and a small amount of a volatilizable halide salt as activeingredients and an inert filler, heating the base metal and powderedpack for a time period sufiicient to cause volatilization of the halogenin the halide salt and to produce deposition of elemental manganese onthe surface of the base metal, then surrounding the manganized basemetal with a powdered pack of a finely ground source of silicon and asmall amount of a volatilizable halide salt as active ingredients and aninert filler, heating the base metal for a time period suflicient tocause volatilization of the halogen in the halide salt and to effect thecreation of an exterior surface layer on the base metal compose-dpredominantly of manganese silicide and a silicide of the base metal.

As previously set forth, conventional silicide coatings on structuralcolumbium alloy substrates are prone to rapid consumption throughoxidation at low (about 1300 F.) temperatures (this tendency issometimes referred to as the silicide pest phenomenon) and at high(about 2000 F. or higher) temperatures. At the latter temperatures arapid oxidation mechanism occurs which, though different from the pestphenomenon, is similar in end result.

Quite unexpectedly, if silicide coatings for columbium and itsstructural alloys are modified with manganese in accordance with thisinvention, the deleterious effects of both the low temperature silicidepest phenomenon and the high temperature rapid oxidation mechanism areessentially overcome. The manganese-modified silicide coatings of thepresent invention are'thus particularly outstanding in their ability toprotect columbium and its alloys from oxidation under a wide variety ofconditions of use and at temperatures that run the gamut up to at leastabout 2200 F. These coatings possess distinctly superior oxidationresistance and superior defect insensitivity up to at least about 2200F. and overcome and counteract the tendency of unmodified ormanganesefree silicide coatings on columbium substrates to fail atcritical temperatures of about 1300 F. and from about 2000 to about 2200F.

In the embodiments forming the examples of this invention, two differentsubstrates were used for the manganese modified silicide coatings. Thesesubstrates are:

(l) Unalloyed columbium, A-inch-diameter rod.

(2) An alloy of Cb-ZOTa-lSW-SMo (additions in percent by weight),representative of columbium-base alloys and hereafter referred to as theAlloy, to Ai-inch-diameter rod.

Other columbium-base alloys could have been used equally well assubstrates to illustrate the new and desirable performance of themanganese-modified silicide coatings for columbium and its alloysdescribed in this specification.

For a clearer understanding of the invention, specific examples of theinvention are set forth in this specification. These examples are merelyillustrative and are not to be understood as limiting the scope andunderlying principles of the invention.

Manganese-modified silicide coatings were applied to these substrates byutilizing a two-cycle pack cementation process, in which the first cyclecomprise-d embedding chemically cleaned and polished specimens to becoated in a manganizing pack of the following mixture:

37 percent by weight of manganese powder 3 percent :by weight of NaClpowder 60 percent by weight of A1 0 powder.

The packs, contained in covered steel cans, were then subjected tovarious thermal treatments in an argon atmosphere at temperaturesranging from 1800 F. to 2200 F. for times from about 1 hour to about 10hours as required to deposit a desired amount of manganese. Successfuldeposition of manganese can also be achieved in accordance with theinvention at temperatures ranging from 1400 F. to 2400 F. During thistreatment, manganese reacted with the substrate to form the compound MMn at the surface (where M represents about the pro portionateingredients as they occur in the substrate). For example, themanganese-rich coating over columbium was CbMn and over the Alloy wasessentially The second cycle comprised embedding the previouslymanganized Cb or Alloy substrates in a siliconizing pack of thefollowing mixture, expressed in weight per gram:

17 percent silicon powder 3 percent N aF powder percent A1 0 powder.

These packs, contained in covered steel cans, were then subjected tovarious thermal exposures in an argon atmosphere at temperatures fromabout 20 00 to about 2400 F. for times ranging from about /2 hour toabout 4 hours as required to form the desired thickness of manganesemodified silicide coating (thicknesses between 3 and 6 mils wereachieved); During this treatment, the siliconrich atmosphere within thepack reacted first with the M M112 compound to generate about equalvolumes of mixed phases based on MnSi and MSi structures. Afterdepletion of the original M Mn phase, silicon reacted with thesubstrate, forming additional MSi phase to achieve the desired totalcoating thickness. An alternative siliconizing procedure was to packpreviously rnanganized substrates in a siliconizing pack of thefollowing weight percent mixture:

5 percent silicon powder 3 percent NaF powder 92 percent A1 0 powder.

Graphite cups were used as containers for this pack mix. SiliconiZin-gkinetics using the lean mixture in graphite cups were about identical tothose using the rich mixture in steel cans when siliconizing wasconducted at 2200 F. Still another procedure, utilizing the rich (17percent silicon) pack in graphite containers resulted in more rapidrates of deposition.

The procedures described above are representative of only one method bywhich manganese-modified silicide coatings can 'be deposited uponcolumbium-base substrates. The superior performance of the coatingabides in the composition and structure of the coating, and is notrestricted to coatings deposited by the pack cementation process.

Representative examples of manganese-modified silicide coatings formingembodiments of this invention are set forth in Table 1, which alsoreports the coating conditions utilized in achieving theserepresentative coatings. In accordance with the invention, it has beendiscovered that the most significant distinguishing characteristic ofthese coatings is the ratio of specific weight gain during manganizingto the specific weight gain during siliconizing. This ratio, whichcontrols the chemical composition and, to a large extent, the structureof the manganese-modified silicide coatings, is set forth in the lastcolumn of Table 1.

TABLE 1.EXAMPLES OF MAN GANESE-MODIFIED SILICIDE COATINGS MauganizingSiliconizing Coating Ratio, Example Substrate Specific Specificthickness, MnzSi 11 Temp., Time, weight Temp, Time, weight mil 8 F. hr.gain, F. hr. gain,

nag/em. mgJcm.

2, 200 10. 5 20. 2 2, 200 0. 65 d 6. 7 4. 3:1 2, 200 3.1 13.6 2, 200 1.17 d 14. 2 4. 2 1:1 2, 200 2 11.2 2,200 4 B 17.1 4.2 1:1.5 2, 200 2 12.4 2, 200 4 e 19. 0 4. 7 1:1. 5 2,200 2 15.8 2,200 4 B 31. 0 7. 0 1:2 2,200 2. 2 11. 5 2, 200 2. 5 i 33. 2 6. 7 1:3 1, 900 7 7. 9 2, 200 2. 539. 2 7. 0 1:5 1, 900 2. 5 4. 6 2, 200 2. 5 i 36. 5 6. 1 1:8

8 Calculated on total weight-gain basis. 6.7 mg.!cm. 1-mil coatingapproximately.

b Ratio of specific weight gain in manganizingzspecific weight gain insiliconizing.

The eifects of this Mn:Si weight ratio upon coating chemistry andstructure can best be understood by using one of the reference examples.The drawing shows the structure of a manganese-modified silicide coatingapplied to the Alloy substrate by manganizing for 2 hours at about 2200"F. followed by siliconizin-g in the rich pack mixture contained in asteel can for 4 hours at about 2200" F. The resulting MnzSi ratio was1:2. Chemical analysis of this type of structure, per-formed by electronbeam microprobe techniques, showed the subsilicide region to beconsistent with an M Si phase. Analysis of the disilicide zone wasconsistent with an MSi phase. In each case, M represents proportionateamounts of columbium, tantalum, tungsten, and molybdenum (i.e., Cb-Ta-15W-5Mo). Neither phase contained more than 2 to 3 percent by weight ofmanganese. The outer, mixed-phase region consisted of the gray-etchingMnSi phase, containing about 15 percent by weight of substrate elements(Cb, Ta, W, Mo), and the manganese deficient MSi phase (the \whitematrix of this region, as shown in the drawing).

In accordance with the invention, the coating structure may becontrolled by controlling the ratio by weight of Mn to Si in themanganese-modified silicide coatings. For example, with higher MnzSiratios (1:15, 1:1, 1.5: 1) the mixed-phase region is predominant, andwith lower ratios (1:3, 1:5, 1:8) the disilicide region is predominant.Similarly, the structure of the mixed-phase region may be controlled byadjustment of variables of the coating process itself. For example, byincreasing the rate of deposition during siliconizing, such as, by usinga rich pack in graphite cups, the MnSi phase can be made to predominate,and the proportionate quantity of MSi phase in the mixed phase regionwill then be less than that shown in the drawing.

In accordance with the invention, and as an unexpected new andbeneficial result, the manganese-modified silicide coatings of thisinvention form a manganese silicate glass when exposed to temperaturesin excess of about 1900 F.; it is believed that the greatly improvedoxidation resistance achieved by the coatings of this invention is inpart due to the formation of this glass. Also unexpectedly andbeneficially, the manganese-modified silicide coatings of this inventionbegin to vitrify at about 1900 R, which is well below the minimum giassformation temperature that has previously been reliably reported to-be2200 F. for the MnO-Si0 system. E. M. Levine, H. F. McMurdie, and F. P.Hall, Phase Diagrams for Ceramists, American Chemical Society, Columbus,Ohio, 1956. Even at temperatures below those at which vitrificationoccurs upon oxidation, however, the manganese-modified silicide coatingsof this invention are distinctly superior in oxida- B Alloy substrate isOb-20IaP15W-5Mo (weight percent additions). d Siliconized in graphitecups using lean pack mixture.

9 Siliconized in steel cans using rich pack mixture.

f Siliconized in graphite cups using rich pack mixture.

tion resistance to unmodified silicide coatings for columbium basesubstrates.

To evaluate the protection afforded to columbium base substrates by themanganese-modified silicide coatings of this invention, cyclic oxidationtests were conducted at temperatures of 1000, 1300, 1600", 1900", 2200",and 2500 F. in ambient air (no forced air flow). During testing,specimens, supported on refractory oxide boats, were inserted in anelectrically heated mufiie furnace preset at the desired temperature.Specimens were removed periodically from the furnace and cooled to roomtemperature for visual examination and weighing, after which they werereturned to the furnace for additional oxidation exposure. Timeintervals for cyclic exposures were as shown in Table 2 below:

TABLE 2 Cycle Time for Cumulative cycle, hr. time, hr.

Testing at each temperature was arbitrarily discontinued after a totalof 100 hours oxidation without failure was achieved.

By oxidizing examples of the manganese-modified silicide coatings ofthis invention at various MnzSi ratios under the program described above(Table 2) at 1300 and 2200 F. temperatures (representative of a low andhigh oxidation temperature), it was possible to establish the optimum orpreferred MnzSi ratios for achieving the new and useful results of theinvention. The results of these runs are set forth in Table .3. As shownin Table 3, the manganese-free silicide coatings over 'columbium and theAlloy (Examples 10 and 11) failed TABLE 3.OXIDATION TESTS OFMANGANESE-MODIFIED SILICIDE 1,300 F. oxidation 2,200 F. oxidation MnzSiExample Substrate ratio Duration Cumulative Duration Cumulative of test,weight of test, weight hr. change, hr. change,

mg./cm 2 mgJcm.

3:1 100 0.7 75 b 4.7 1:1 100 0.3 100 b 3.4: 1:1. 5 100 H 7.4 100 5. 51:1. 5 100 0. 1 100 3. 7 1.3 100 0. 5 100 5. 8 1:5 100 1.1 100 d 12.61:8 50 d 16.0 1:1 No test No test 100 b 1.4 i 0 100 33. 4 Co1umbium f 020 75 d 30.4

B Alloy is Cb-20Ta-15W-5Mo. b Excess glass absorbed by support. Actualweight change higher.

0 One large piece of coating spalled. Overall performance unimpaired. dGross uniform oxidation of coating.

e Gross failure by local defect or powdering of coating.

i Manganese-free silicide coating.

The behavior of various manganese-modified silicide coatings oncolumbium and alloys thereof is summarized below:

(1) In coatings containing only about 1 part by weight of manganese to 8parts by weight silicon, the manganese modification has little effect.At both temperatures (l300 and 2200 F.) the coatings behave as thoughalmost no modification had been achieved. (Example ,8.)

(2) Coatings containing at least 1 part of manganese to 5 parts ofsilicon consistently protect the substrates for at least 100 hours at1300 F. (Examples 1, 2, 3, 4, 6, 7.) V

(3) A coating containing 1 part of manganese to 5 parts of siliconoxidizes at a lower rate than manganese-free silicide coatings for to 75hours at 2200 F., but thereafter oxidizes rapidly in a mannercharacteristic of manganese-free silicide coatings. The limited amountof glass-forming phase, MnSi, is not sufficient to be protective formore than about 75 hours. (Example 7.) However, even this amount ofprotection represents an appreciable advantage over manganese-freesilicide coatings.

(4) A coating containing about 1 part of manganese to 3 parts of siliconis fully protective for 100 hours at 2200 F. However, after the100-hour-test the surface of the specimen exhibits little glaze, and itis apparent that life of the coating beyond 100 hours would be limited.(Example 6.)

(5) Coatings containing about 1 part of manganese to 1 /2 parts ofsilicon are protective for 100 hours at 2200 F. Weight gains during thefirst 4 /2 hours are high (about 3 /2 mg./cm. as the protective glass isformed. During the last 95 hours of a 100 hour test at 2200 F., weightgains are very low, about 2 mg./cm. showing the excellent protection ofthe glass oxidation product. After completing 100 hours at 2200 F.,appreciable quantities of MnSi phase are still present in themicrostructures of the coatings. These coatings thus exhibit acapability for protection substantially greater than 100 hours at 2200F. (Examples 3 and 4.)

(6) Coatings containing about equal weights of manganese and silicon arealso fully protective for hours at 2200 F. However, the amount of glassformed may be excessive; during the test it ran off the specimens andwas absorbed by the supporting refractory oxide boats. Weight changesreported in Table 3 are thus unrealistically low. The excessive amountof glass formed in general is undesirable, as it is not required foradequate protection. Despite excessive glass formation, however,appreciable quantities of MnSi phase are still present in themicrostructures of the coatings after 100 hours oxidation at 2200 F.These coatings thus indicate a capability for protection ofsubstantially greater than 100 hours at 2200 F. (Examples 2 and 9.)

(7) A coating containing 3 parts of manganese to 1 part of siliconexhibits the MSi phase only in the mixed phase region, and although itendures for 75 hours before failure, it is not adequately protective at2200 F. An excessive amount of glass is formed. (Example 1.)

Accordingly, within its scope this invention include manganese-modifiedsilicide coatings for columbium-base materials, in which the MnzSiratios by weight are from about 3:1 to 1:5. Such coatings are able toprotect columbium alloy substrates for periods of time greater than 100hours both at temperatures of about 1300", at which the disilicide pestphenomenon occurs, and at temperatures of about 2000 to about 2200 F.,at which a rapid oxidation mechanism occurs. They are thus greatlysuperior to unmodified or manganese-free silicide coatings. Performanceof coatings in the examples forming embodiments of this inventiondiscloses that intermediate ratios of from about 1:1 to 1:3 areparticularly outstanding and that a ratio of about 1:1.5 is preferred.

The oxidation behavior of typical manganese-modified silicide coatingsfor columbium-base materials has been further clarified by oxidation ofnominal 5-mi1 thick, 1:1.5 MnzSi ratio coatings at temperatures of1000", 1600, 1900, and 2500 F. Test results on such coatings at thenamed temperatures are summarized in Table 4. Table 4 characteristicglass. systems to thermal shock, the boat was placed in an elec- JA-ihch orifice in an 80 p.s.i. air line.

TABLE 4.GHARAGTERIZATION OF THE BEHAVIOR OF S-MIL-THICK,

111.5 Mn-Si RATIO COATINGS OVER COLUMBIUM AND Cb-20'la-15W-5Mo ALLOYDURING CYCLIC OXIDATION IN AIR AT VARIOUS TEMPERA- TURES OxidationDuration Cumulative Example Substrate Temp. of Test, eight Remarks F.hr. Change, mgJemfl 1, 000 100 0.2 1,000 100 0.7 1, 300 100 0. 6 Minoroxide spalling. 1,300 100 7.4 Piece of coat g spelled. 1,600 100 1.3 1,000 100 1.3 Alloy 1, 900 100 3. 8 Columbium 1, 900 100 4. 6 Alloy 2, 200100 a. s Columbium..- 2, 200 100 5. Alloy- 2, 500 75 4.8 Failed at localdefeet in 100 hours. columbium... 2,500 75 Complete oxidation.

also includes the results of 1300 and 2200 F. testing re- TABLE portedpreviously. At 1000, 1600, and 1900 F., oxidation was mild and thecoatings easily protected the subi tan Cooling Rate, strates for 100hours. Oxidation resistance of the mangain. degrees/sec. nese-modifiedcoating over columbium-base materials at these temperatures was aboutthe same as that exhibited g 33 by unmodified or manganese-free silicidecoatings, which also perform well at these temperatures. At 2500 F., themanganese-modified silicide coatings investigated were limited to acapability of protecting the columbium-base substrates for about 50-75hours. Essentially the same behavior was exhibited by unmodified ormanganese-free silicide coatings at this temperature.

After the tests described above had shown the superior performance ofthe manganese-modified silicide coated columbium-base materials duringcyclic oxidation, tests were conducted on additional examples orembodiments of the invention to demonstrate resistance of the coatedsystems to thermal shock, intentional defects, and high velocity gaseouserosion, as well as their ability to achieve self-healing ofintentionally introduced defects. (Defects to a depth of at least 5 and6 mils in both columbium and the Alloy respectively were healed within 3hours at 2200 F., and those to a depth of at least 9 mils in the Alloywere healed after an additional exposure of 17 hours including 1 cycleat 1 /2 hours and 1 cycle at 15 /2 hours.) For most of these tests, rodspecimens of both unalloyed columbium and the Alloy were coated in amanner described previously to achieve nominal S-mil-thickmanganose-modified silicide coatings with nominal Mn:Si ratios between121.6 and 1:1.2.

confines of the boat. Before thermal shock testing, specimens werepreoxidized for 1 hour at 2200" F. to form the To test the resistance ofthe coated tric fired furnace at 2200 F. In about 1 minute the assemblywas about at that temperature, but was allowed an additional l to 4minutes for thermal stabilization (or a total of 2 to 5 minutes heatingat 2200 F.). The boat containing the specimens was then removed from thefurnace and the specimens were immediately (about 1 second elapsed time)given a blast of air issuing from a The rate of cooling was governed bythe distance between the orifice and the specimens, and was measured asshown in Table 5 below:

TABLE 6.HISTORY OF THERMAL SHOCK TESTING OF MANGANESE-MODIFIED SILIGIDECOATING ON A COLUMBIUM-BASE ALLOY Operation Observation 13 cycles, lowcooling rate-" 7 cycles, high cooling ratc 15 min. soak at 2,200 F 20cycles, high cooling rate hr. soak at 2,200 F 15 cycles, high coolingrate (on specimen arbitrarily removed for metallography; tests continuedon other specimen). 24 hr. soak at 2,200 F., plus water quench. 23 hr.soak at 2,20[) F., plus normal air cool.

. No change in appearance.

Do. Do.

No efiects from thermal shock. N 0 change in appearance.

Slight spalling of oxide at edge.

Total time at 2,200 31-153 hr. Cycles, low coollng rate-13. Cycles, highcooling rate-42. Cycles, water quench-l. Total thermal shock cycles56.

was shown that the manganese-modified silicide coating over arepresentative columbium alloy can sustain repeated quenching from 2200"F. to about room temperature without failure. Furthermore, as some ofthe thermal shock cycles were conducted after about hours oxidationexposure, the thermal stability of the coating regarding thermal shockresistance is excellent. Microstructurally, the coatings showed nodetrimental effects that could be attributed to thermal shock.

Defects were intentionally introduced in several manganese-modifiedsilicide coated columbium and Alloy specimens by two methods:

(1) Coated specimens were indented with a diamond pyramid hardnessindenter (Vickers) with loads up to 50 kilograms. Maximum depths of theindentations were 4.9 mils in the caseof coated columbium, and 2.4 milsin the case of the coated Alloy (reflect- 13 ing primarily the greaterhardness of the Alloy substrate compared with unalloyed columbium).Although no fracture of the coating as a result of Vickers indentationwas apparent macroscopically, metallographic examination showed completefracture paths through the coatings beneath the indentations, providingpotential free access of air to the substrate. (2) Coated specimens werenotched with a file to depths as shown in Table 7 below:

Examples of Vickers indented specimens were exposed to oxidation at -1600 F. (below the glass forming temperature) and 2200 F. (within theglass forming temperature range) for times of 75 hours and 100 hours,

respectively, with no failures observed. Good defect insensitivity wasthus demonstrated for the manganesemodified silicide coatings overcolumbium base materials. File-notched specimens were oxidized at 2200F. to assess the self-healing capability of the glass coating instagnant air. After the first '(l /z hours) oxidation cycle,

substrate oxidation was observed at all three notches where the coatinghad been penetrated (Notches 2, 3,

and 4). During the second cycle (1 /2 hours), self-healing occurred atNotch 2 of specimens of both systems (coated colurnbium and Alloy), butnot at the two deeper notches. No additional change was noted during thethird (1 /2 hours) cycle. However, during the fourth cycle (15 /2hours), self-healing occurred :at' Notch 3 of the coated Alloyrestricting further substrate oxidation at this site. This phenomenonwas not observed, however, on the coated colu-mbium example. This testthus demonstrates the self-healing capability of the manganesernodifiedsilicide coating over columbium-base substrates at temperatures at whichthe glass is formed upon oxidation.

A special shaped specimen was machined from the Alloy and coated with a3-mil thick, 1:1 MnzSi ratio manganese-modified silicide coating. Thisspecimen was then subjected to a special gas erosion test designed tosimulate the gas chemistry and velocity conditions existing in theturbine of 'a turbojet engine operating at 2200 F. The test consisted of8 cyclic exposures to the high velocity gas, each of -hours duration,for a total test time of 40 hours. Through 35 hours accumulated time,the surface of the specimen exhibited the normal glassy appearance, buton the last cycle, the appearance indicated depletion of the MnSiglass-(forming plase. This was confirmed by metallographic examination,which showed none of the protective glass-forming, mixed phase regionremaining.

This erosion test showed that although high velocity gas significantlyincreases the nate of depletion of the protective glass-forming mixedphase region compared with a static atmosphere, protection attributableto the manganese modification is still appreciable.

In accordance with the invention, it has thus been shown thatmanganese-modified silicide coatings for columbium base alloys exhibitgreatly superior oxidation resistance to unmodified or manganese-freesilicide coatings over the same substrates. Silicide coatings containingweights of manganese from about 3 times to /s the weight of siliconpresent exhibit this superiority. Coatings containing weights ofmanganese from about equal to, to /3 the weight of silicon present areparticularly outstanding, while coatings containing a ratio by weight of1:1.5, manganese to silicon, are most preferred. It has been furtherdemonstrated that such manganesemodified silicide coatings :forcolumbium and its alloys exhibit excellent resistance to thermal shockfail-ure, ex cellent defect insensitivity at temperatures both higherand lower than the glass-forming temperature, at least moderateself-healing capabilities at temperatures above the glass-formingtemperatures, and significant resistance to high velocity gas erosion.

The invention in its broader aspects is not limited to the specificdetails shown and described, but departures may be made from suchdetails within the scope of the accompanying claims without departingfrom the prin ciples of the invention and without sacrificing its chiefadvantages.

What is claimed is:

1. An article of manufacture having good stressrupture strength at hightemperatures, high temperature oxidation resistance, and resistance tocyclic fatigue failure, which comprises a core of metal selected fromthe group consisting of columbium and columbium base alloys, the articlehaving a thermal shock failure resistant, defect insensitive,self-healing oxidation resistant surface layer consisting essentially ofa mixture of manganese silicide and a silicide of the metal core.

2. The invention as defined in claim 1, in which the surface layer has amanganese to silicon ratio by weight of from about 3:1 to 1:5.

3. The invention defined in claim 1, in which the surface layer has amanganese to silicon ratio by weight from about 1:1 to 1:3.

4. The invention as defined in claim .1, in which the surface layer hasa manganese to silicon ratio by weight of about 1: 1.5.

S. An article of manufacture having good stress-rupture strength at hightempenatures, high temperature oxidation resistance and resistance tocyclic fatigue failure, which comprises a core of metal selected fromthe group consisting of colum-bium and columbium base alloys, thearticle having a thermal shock failure resistant, defect insensitive,self-healing oxidation resistant coating comprising: a surface zoneconsisting essentially of a mixture of manganese silicide and adisilicide of the metal core and a subzone essentially beneath thesurface zone and consisting essentially of a disilicide of the metalcore.

6. A coated metal body comprising a substrate selected from the groupconsisting of columbium and columbium base alloys and having aprotective coating at least on that part of the substrate that isexposed to attack by oxygen at high temperatures, the coating containinga mixture of manganese silicide and a silicide of the metal body andbeing oxidation resistant, thenrnal shock failure resistant, defectinsensitive and self-healing at high temperatures.

7. The invention as defined in claim 6, in which the coating has amanganese to silicon ratio by weight of from about 3:11 to 1:5.

8. The invention as defined in claim 6, in which the coating has amanganese to silicon ratio by Weight [from about 1:1 to 1:3.

9. The invention as defined in claim 6, in which the coating has amanganese to silicon ratio by weight of about 1:15.

10. Acoated metal body comprising a substrate selected from the groupconsisting of columbium and columbium base alloys and having aprotective coating at least on that part of the substrate that isexposed to attack by oxygen at high temperatures, the coatingcomprising: a surface zone containing a mixture of manganese silicideand a disilicide of the metal body and a subzone essen- .tially beneaththe surface zone and containing a disilicide body, comprising asubstrate selected from the group consisting of columbi-um and columbiumbase alloys and .having an exterior exposed layer composed predominantlyof -a mixture of manganese silicide and a silicide of the metal body,the body being characterized by a good stress-rupture strength at hightemperatures and a high resistance to oxidation at temperatures up to atleast about 2200 F.

12. The invention as defined in claim 11, in which the exterior exposedlayer has 'a manganese to silicon ratio by weight of from about 3:1 to1:5.

.13. The invention as defined in claim 11, in which exterior exposedlayer has a manganese to silicon ratio by weight from about 1:1 to 1:3.

14. The invention as defined in claim 11, in which the exterior exposedlayer has a manganese to silicon ratio by weight of about 1:15.

15. A coated metal body having (good stress-rupture strength at hightemperatures, good oxidation resistance at high temperatures andresistance to cyclic tatigue "failure, which comprises a core of metalselected from vthe group consisting of columbium and alloys thereof,

the body having a thermal shock failure resistant, defect insensitive,self-healing oxidation resistant coating, the coating comprising: amixed phase surface zone consisting essentially of a mixture ofmanganese silicide and a disilicide of the metal core, an intermediateregion beneath the surface zone consisting essentially of a disilicideof the metal core, and a diffusion zone adjacent the metal coreconsisting essentially of at least one subsilicide of the metal core.

16. The invention as defined in claim 15, in which the coating has amanganese to silicon ratio by weight of from about 2:1 to 1:5.

17. The invention as defined in claim 15, in which the coating has amanganese to silicon ratio by weight of from about 1:1 to 1:3.

18. The invention as defined in claim 15, in which the coating has amanganese to silicon ratio by weight of about 1:15.

19. The process of coating a fabricated base metal which processcomprises surrounding a base metal selected from the group consisting ofcolumbiurn and alloys thereof with 'a powdered pack of a finely groundsource of manganese and a small amount of a volatilizaible halide saltas active ingredients and an inert filler, heating the base metal andpowdered pack for a time period sufficient to cause volatilization ofthe halogen in the halide salt and to produce deposition of elementalmanganese on the surface of the base metal, then surrounding themanganized base metal with a powdered pack of a finely ground source ofsilicon and a small amount of a volatilizable halide salt as activeingredients and an inert filler, heating the base metal for a timeperiod sufficient to cause volatilization of the halogen in the halidesalt and to effect the creation of an exterior surf-ace layer on thebase metal composed predominantly of manganese silicide and a silicideof the base metal.

20. The process of treating a metal from the group consisting ofcolumbium and alloys thereof to render the surface of the metalresistant to oxidation at high temperatures, that includes, heating themetal to a temperature between from about 1400" to 2400 F. in anon-oxidizing atmosphere and in surface contact with a powdered mixtureof manganese, an inorganic halide, and an inert refractory material, andsubsequently heating the metal to a temperature of from about 2000 toabout 2400 F. in a non-oxidizing atmosphere with the surface of themetal in contact with a powdered mixture of silicon,

an inorganic halide, and an inert refractory material to form thereby aprotective surface coating on the metal consisting essentially ofmanganese silicide and a silicide of the base metal.

21. A method of producing a high temperature oxidation resistant,thermal shock failure resistant, defect :insensitive, self-healingcoating surface layer on a metal article formed of a substrate selectedfrom the group con- .sisting of col-umbium and col-umbium sbase alloys,the

coating surface layer consisting essentially of a mixture of manganesesilicide and a silicide of the metal substrate; the method comprisingthe steps of: enclosing the article in a manganiZin-g pack of powderedmaterial containing a source of manganese and a small amount of avolatilizable halogen generating substance as essential activeingredients and an inert filler, heating the article in the pack to atemperature higher than that causing volatilization of the halogensubstance, and maintaining this temperature for a discrete interval oftime to efiect the deposition of manganese onto the surface of thearticle, then enclosing the manganized article in a siliconizing pack ofpowdered material containing a source of silicon and a small amount of avolatilizable halogen generating substance as essential activeingredients and an inert filler,

heating the article in the pack to a temperature higher than thatcausing volatilization of the halogen substance to effect the creationof an exterior surface layer composed predominantly of a mixture ofmanganese silicide and a silicide of the substrate.

22. The method as defined in claim 21, in whieh'the metal article duringthe manganizing step is heated in the pack to a temperature of fromabout 1400 F. to 2400- F.

23. The method as defined in claim 21, in which the rnetal article isheated in the silic-onizing pack to a temperature of from about 2000 toabout 240051 for from about one-halt hour to about 4 hours.

References Cited by the Examiner UNITED STATES PATENTS 3,015,579 1/1962;Commanday et al. 117-71 3,168,380 2/1965 Bradley et a1. 117131 X3,219,474 11/1965 Priceman et a]. 1171 3l X 3,249,462 5/1966 Jung et a1.117106 ALFRED L. LEAVI'IT, Primary Examiner.

A. GOLIAN, Assistant Examiner.

19. THE PROCESS OF COATING A FABRICATED BASE METAL WHICH PROCESSCOMPRISES SURROUNDING A BASE METAL SELECTED FROM THE GROUP CONSISTING OFCOLUMBIUM AND ALLOYS THEREOF WITH A POWDERED PACK OF A FINELY GROUNDSOURCE OF MANGANESE AND A SMALL AMOUNT OF A VOLATILIZABLE HALIDE SALT ASACTIVE INGREDIENTS AND AN INERT FILLER, HEATING THE BASE METAL ANDPOWDERED PACK FOR A TIME PERIOD SUFFICIENT TO CAUSE VOLATILIZATION OFTHE HALOGEN IN THE HALIDE SALT AND TO PRODUCE DEPOSITION OF ELEMENTALMANGANESE